This invention relates to alkaline cells and more particularly to alkaline cells having zinc electrodes.
Nickel-zinc cells offer the advantages of high EMF, high energy per unit of weight and volume, and low cost. In recent years the nickel-zinc battery has been receiving increased attention by the Energy Department as a candidate for use in electric vehicles. The Defense Department is also interested in nickel-zinc cells for use in electric propulsion, aircraft, combat vehicles, and electronics communication batteries. Obviously, long life nickel-zinc batteries are necessary for these applications.
The most commonly employed separators have a cellulosic structure in the form of cellophane (regenerated cellulose by the Xanthate process) or fibrous sausage casing (prepared from cotton linters). These separators are catonic-diffusion controlled semipermeable membranes; i.e., during the charge and discharge processes they favor the diffusion of hydrated potassium ions.
Tests show that shorting by zinc penetration through diffusion-type separators usually results in partial shorts; zincate ions have difficulty in diffusing through the highly tortuous interstices of these separators. Zinc dendrites that reach the nickel sinter of the nickel cathode electrode are apparently dissolved by the vigorous local action between zinc and the nickel sinter at a rate equal to or greater than the regrowth of new zinc dendrites into the nickel cathode compartment. The net result is a minute reduction in potential between the zinc anode and nickel cathode.
A serious disadvantage of solely using several layers in the diffusion-type separator is that they usually cause the formation of large concentration gradients, particularly when employed as several layers. Large concentration gradients produce electro-osmotic pumping effects which promote washing and shape change of the zinc anodes with the resulting loss of capacity by the cell.
On the other hand, mass transport-type membranes minimize the formation of large concentration gradients. Unfortunately, in the case of these mass-transport separators, zincate ion mobility is not impeded, and zinc dendrites are generated in the open micropores. These zinc dendrites then reach the nickel cathodes as a front within a relatively short time. The resulting short is massive between the zinc anode and the nickel cathode.